The present invention relates a current monitoring system and, more particularly, to a user configurable branch circuit monitor with a memory for storing transducer characteristics.
The total power consumption of a building or other facility is typically monitored by the electric utility with a power meter located between the utility's distribution transformer and the facility's power distribution panel. However, in many instances it is desirable to sub-meter or attribute the facility's power usage and cost to different occupancies, buildings, departments, or cost centers within the facility or to monitor the power consumption of individual loads or groups of loads, such as motors, lighting, heating units, cooling units, etc. These single phase or multi-phase electrical loads are typically connected to one or more of the branch circuits that extend from the facility's power distribution panel. While a power meter may be installed at any location between a load and the distribution panel, typically a power meter capable of monitoring a plurality of circuits is installed proximate the power distribution panel to provide centralized monitoring of the various loads powered from the panel.
Flexibility has favored adoption of digital current and power meters, known as branch circuit monitors, incorporating data processing systems that can monitor a plurality of circuits and determine a number of parameters related to electricity consumption. A digital power meter for measuring electricity consumption by respective branch circuits comprises a plurality of voltage and current transducers that are periodically read by the meter's data processing unit which, in a typical digital power meter, comprises one or more microprocessors or digital signal processors (DSP). The data processing unit periodically reads and stores the outputs of the transducers sensing the magnitudes of the current and voltage and, using this sample data, calculates the current, voltage, power, and other electrical parameters, such as active power, apparent power and reactive power that quantify electricity distribution and consumption. The calculated parameters are typically output to a display for immediate viewing and/or transmitted from the meter's communications interface to another data processing system, such as a building management computer for remote display or further processing, for example formulating instructions to automated building equipment.
While the voltage transducer(s) of a digital power meter can be interconnected anywhere in the wiring that connects the supply and a load, including at the load's terminals, interconnection of voltage transducers and the facility's wiring is facilitated by wiring connections in the power distribution panel. The power distribution panel provides a convenient location for connecting the voltage transducers because typically each phase of the power is conducted by a separate bus bar in the power distribution panel and the voltage and phase is the same for all loads attached to the respective bus bar. The voltage transducers of digital power meters commonly comprise a voltage divider network that is connected to a conductor in which the voltage will be measured.
The current transducers of digital power meters typically comprise current transformers that encircle the respective power cables that interconnect each circuit to the bus bar(s) of the distribution panel. To measure the power consumed by the plurality of loads making up a facility, a current transformer must be installed in each branch circuit connecting the load(s) to the distribution panel. A current transformer typically comprises multiple turns of wire wrapped around the cross-section of a toroidal core. The power cable, conducting the load current, is passed through the aperture in the center of the toroidal core and constitutes the primary winding of the transformer and the wire wrapped around the cross-section of the core comprises the secondary winding of the transformer. Current flowing in the primary winding (primary current) induces a secondary voltage and current in the secondary winding which is quantitatively related to the current in the primary winding. The secondary winding is typically connected to a resistor network and the magnitude of the primary current can be determined from the amplitude of the voltage at the output of the resistor network.
Accurate measurement of electric power also requires compensation for error introduced by the transducers comprising the power meter. For example, the secondary current of a current transformer is ideally equal to the load current flowing in the power cable (the primary winding) divided by the number of turns in the secondary winding. However, magnetization of the core of the transformer produces ratio and phase errors which vary with the magnitude of the current being measured and the configuration of the particular transformer, including factors such as core material and turns ratio. Typically, a power meter is configured for use with a particular transducer and error compensation factors, ascertained by experimentation with sample transducers, are built into the meter and applied by the data processing unit during calculation of the meter's output.
However, a user may desire to use a different transducer than that specified for use with a particular meter or several different transducers may be required because of a wide variation in the magnitude of the current in the various branch circuits or it may be necessary to replace one or more transducers during use. Using transducers that are not matched to the meter results in inaccurate readings from the meter. What is desired, therefore, is a branch circuit monitor providing flexible construction, simplified installation and improved serviceability.